I constantly see people complaining about Americans writing British characters and screwing up the manners of speaking, terms, etc, which I agree is annoying, but it's even more frustrating to me when it's the other way around, probably because there are so many American tv shows and whatnot that are globally syndicated.
No normal, average-joe American uses terms like "mum," "mate," "flat," or "arse."
I've only read the first book, but
Fifty Shades of Grey is the most infamous example I can think of because the MFC is
constantly using Brit terms when she's
supposed to be American. There was a few times where she mentioned how "smart" her outfit looked or whatever, and at first I thought, "oh, she means this outfit makes her look intelligent. Okay," and then I came across it a few more instances and was just like, "THIS MAKES NO FUCKING SENSE
" before I eventually realized it was a Brit thing